The present invention relates to latent curing agents and accelerators for epoxy resins including water-based compositions, especially one-component water-based epoxy compositions. xe2x80x9cLatentxe2x80x9d curing agents are those curatives that in a formulated system remain inactive under normal ambient conditions but react readily with epoxy resin at elevated temperatures . xe2x80x9cAcceleratorsxe2x80x9d are those materials that accelerate the reaction between epoxy resin and a curing agent. xe2x80x9cOne componentxe2x80x9d epoxy compositions are typically a blend of epoxy resin, curing agent and accelerator as well as additives and fillers.
Current one-component water-based epoxy compositions contain dicyandiamide as a latent curing agent dispersed in aqueous epoxy emulsions. These compositions offer excellent shelf life but require very high temperature to cure. Water-soluble accelerators such as imidazoles can be used with dicyandiamide to increase reactivity, however such accelerators adversely affect shelf stability of compositions.
Commercially available latent curing agents offer long shelf stability and good low temperature cure in 100% solids compositions. However, they are not useful in water-based compositions unless a co-solvent is used to dissolve the latent curing agent. The use of solvents increases VOC (volatile organic component) content and adversely affects the shelf stability of the epoxy composition.
There is a need for a water-soluble accelerator for dicyandiamide latent curing agent in heat curable water-based epoxy compositions.
There is a need for a one-component 100% solids epoxy composition which is cured by dicyandiamide and offers a good balance of low-temperature cure and shelf stability.
There is a need for a one-component water based epoxy composition which is cured by dicyandiamide and offers a good balance of low-temperature cure and shelf stability.
U.S. Pat. No. 3,329,652 discloses curing polyepoxides with acid anhydrides using imidazole salts as activators for the acid anhydride.
U.S. Pat. No. 3,356,645 and 3,418,333 disclose curing polyepoxides with imidazole salts.
U.S. Pat. No. 3,746,686 discloses curable epoxy resin compositions comprising a polyepoxide and a salt of a polycarboxylic acid or anhydride and an imidazole.
U.S. Pat. No. 3,755,253 discloses catalyzing the diaminodiphenylsulfone cure of polyepoxides with an imidazole salt.
T. Kamon, et al, xe2x80x9cCuring of Epoxy Resins. VI. Curing of Epoxy resins with Acid Salts of Imidazolesxe2x80x9d, Shikizai Kyokaishi (1977), 50 (1), pp 2-7 discloses the study of the curing of epoxy resins with alkyl carboxylic acid and phosphoric acid salts of some imidazoles.
JP 58083023 describes a latent epoxy hardener prepared by placing 2-hepta-decylimidazoline in water, adding orthophosphoric acid, mixing for 10 minutes, filtering and drying in vacuo. xe2x80x9cEpoxy Dispersion In Adhesive Applicationsxe2x80x9d, Adhesives Age, May 1995, pages 34-37, discloses the use of 2-methylimidazole and dicyandiamide in water-based epoxy compositions.
The present invention is directed to imidazole phosphate salts as accelerators for dicyandiamide latent curing agents and their use in one-component heat curable epoxy compositions, especially water-based compositions. The salts are the reaction product of an imidazole and phosphoric acid.
Illustrative of the imidazoles useful for making the accelerators are those compounds of the structure A 
where R1, R2, R3 and R4 are independently hydrogen, a C1-C18 alkyl, a phenyl or a C7-C12 alkylaryl group, these substituent groups optionally containing a functionality such as but not limited to ether, alcohol, amine, nitrile, mercaptan and thiol.
The reaction of these imidazoles with phosphoric acid affords the dihydrogen phosphate salt, or biphosphate, of the following structure B 
where R1, R2, R3 and R4 are as described above.
The invention provides:
accelerators for dicyandiamide curative in heat cured epoxy compositions.
a water-soluble accelerator for dicyandiamide cured heat curable water-based epoxy compositions.
one-component 100% solids epoxy compositions comprising an imidazole-phosphate salt, dicyandiamide and an epoxy resin which offer a good balance of low-temperature cure and extended shelf stability.
one-component water based epoxy compositions comprising an imidazole-phosphate salt, dicyandiamide and an epoxy resin which offer a good balance of low-temperature cure and shelf stability.
The imidazole phosphate salts are water soluble and accelerate epoxy reactions with dicyandiamide.
The invention relates to imidazole (IM) salts of phosphoric acid and their use as accelerators for dicyandiamide in curing epoxy resins. (Phosphoric acid is also known as orthophosphoric acid and is commercially available as 85% phosphoric acid.) While any imidazole that forms a salt with phosphoric acid, which salt is preferably water soluble for water-based compositions, can be suitably used, the preferred imidazoles for use in the present invention are those having the structure A. Salts having structure B are the monobasic salts, or dihydrogen phosphate salts, formed by the reaction of 1 mole of an imidazole with 1 mole of phosphoric acid according to the reaction scheme: 
where R1, R2, R3 and R4 are independently hydrogen, a C1-C18 alkyl, preferably a C1 to C3 alkyl, a phenyl or a C7-C12 alkylaryl, preferably C7 to C8 alkylaryl group. Optionally, such alkyl groups may contain a functionality such as but not limited to ether, alcohol, amine, nitrile, mercaptan and thiol.
Suitable alkyl groups include, for example, methyl, ethyl, n- and isopropyl, n-, iso-, sec- and tert-butyl, 2-ethyhexyl, octyl, decyl, dodecyl and heptadecyl.
Suitable alkylaryl groups include tolyl, xylyl and ethylphenyl.
The preferred imidazoles for reacting with phosphoric acid include imidazole, 1-methyl imidazole, also called N-methylimidazole, 2-phenylimidazole and 2-methyl imidazole.
The stoichiometry employed in the synthesis of the phosphate salts can be any combination, e.g., ranging from 0.1 moles to 5.0 moles of imidazole and 0.1 moles to  greater than 5.0 moles of phosphoric acid. Generally, the imidazole and the phosphoric acid are reacted in a 0.9 to 1.1 molar ratio, preferably a 0.95 to 1 molar ratio. Typically the commercially available 85% orthophosphoric acid is employed but any concentration can be used in this invention. The reactions can be conducted with or without the use of solvent. The solvents may be but are not limited to water, methanol, ethanol, THF and the like. Any solvent which will dissolve one of the reactants or the product may be used. Any order of addition may be used and the reaction may be conducted at any temperature or pressure desired as they are not critical to making the salt. The preferred method of synthesis is to dissolve the imidazole in methanol and add the orthophosphoric acid slowly to the imidazole solution. The resulting precipitated salt is collected by filtrating, washing with methanol and air drying.
The imidazole/phosphoric acid salts can be used as accelerators for the latent curing agent dicyandiamide in one-component epoxy adhesives, decorative and protective coatings including powder coatings, filament winding, printed circuit board and like epoxy applications. Typically, 0.5 to 10 parts by weight (pbw) dicyandiamide are used in the epoxy composition per 100 pbw epoxy resin, preferably 2 to 6 pbw of dicyandiamide.
The imidazole-phosphate accelerator with the dicyandiamide curing agent is combined with an epoxy resin which is a polyepoxy compound containing more than one 1,2-epoxy groups per molecule. Such epoxides are well known in the epoxy art and are described in Y. Tanaka, xe2x80x9cSynthesis and Characteristics of Epoxidesxe2x80x9d, in C. A. May, ed., Epoxy Resins Chemistry and Technology (Marcel Dekker, 1988). Examples include those epoxides disclosed in U.S. Pat. No. 5,599,855 (Col 5/6 to 6/20), which is incorporated by reference. The preferred polyepoxy compounds are the diglycidyl ethers of bisphenol-A, the advanced diglycidyl ethers of bisphenol-A, the diglycidyl ethers of bisphenol-F, and the epoxy novolac resins. Both liquid epoxy resins and solid epoxy resins are suitably used in the one component epoxy compositions. Powder coating compositions would comprise a solid epoxy resin, imidazole phosphate salt and dicyandiamide.
Generally, an effective amount of the imidazole phosphate salts for accelerating the dicyandiamide curing of the epoxy resin is used. As an accelerator for dicyandiamide, 0.5 to 10 parts by weight (pbw) of the phosphate salt would be used per 100 pbw of epoxy resin.
Compositions prepared from phosphoric acid salts of an imidazole, dicyandiamide and epoxy resins can be formulated with a wide variety of ingredients well known to those skilled in the art of coating formulation, including solvents, fillers, pigments, pigment dispersing agents, rheology modifiers, thixotropes, flow and leveling aids, defoamers, etc.
While epoxy compositions comprising 1 to 90 wt % organic solvents or 100% solids can be used, it is preferred the epoxy composition be water-based, i.e., an aqueous epoxy system containing 20 to 80 wt % solids, preferably 50 to 60 wt % solids.
One component epoxy compositions of this invention can be applied as coatings by any number of techniques including spray, brush, roller, paint mitt, and the like. Numerous substrates are suitable for application of coatings of this invention with proper surface preparation, as is well understood in the art. Such substrates include but are not limited to many types of metal, particularly steel and aluminum, as well as concrete.
One component epoxy coating compositions of this invention can be applied and cured at elevated temperatures ranging from about 80xc2x0 C. to about 240xc2x0 C., with cure temperatures of 120xc2x0 C. to 160xc2x0 C. preferred.